We need someone who can modify an existing website according to a new idea -- 2

I have an existing website which I need to modify according to a new idea. I plan to use PHP as the programming language for this. I already have an existing website that needs to be modified and am looking for someone to help make these changes by adding a new feature. The modifications should be done in such a way that it does not interfere with the current functionality or design of the website. If possible, I would like to get the job done quickly and accurately.